A variety of manufacturing processes handle continuous materials under tension. Wire, rope, thread, fiber optic filaments, films, paper webs, metal foils, ribbon, and other continuous materials are commonly processed under tension. The material may be handled under tension during the initial phases of processing, during intermediate phases and/or in the final phase of processing into a finished product. The uniformity of the finished product in these processes may depend upon the uniformity of the tension of the material as it is processed. The processing of materials having low tensile strengths requires maintaining process tension levels within narrow ranges to prevent breakage of the material and the corresponding loss of process productivity.
Automated process controllers such as Proportional, Proportional+Integral (PI), and Proportional+Integral+Derivative (PID) controllers are used to control material tension during processing. PI, and PID controllers, calculate an error signal as the difference between a parameter set point and the measured value of the parameter. The output of the controller is then modified according to the error signal and one or more “gains” of the controller. The output is a function of the error signal and the gains. The calculation of the output may also involve constant terms. In instances where the values of controller gains are fixed, the gains are constant terms and the output is a function of the error signal. This is an iterative, feedback loop, process. The controller gains are named for their relationship to how the error signal is used. The proportional gain is used to compute output correction in proportion to the error signal. The integral gain is used to compute output correction according to the sum, or integral, of a value derived from the error signals. The derivative gain is used to compute output correction in relation to the rate of change, or derivative, of the error signal, or another signal such as the loop feedback.
Typical prior art control methods are “tuned” or optimized, by selecting appropriate controller gain values to achieve a desired process stability and rate of response. The controller gain values may be adjusted by process operators, these adjustments are manual and are related to changes in the incoming material or the process equipment performance. In some methods, the values of the controller gains are scheduled to change with the diameter of the roll of material as it is wound or unwound depending upon the specifics of the process being controlled.
Typical control methods do not provide adequate tension control at low process speeds. Typical loop tuning methods result in tension control over a speed range from a maximum speed to approximately one-tenth the maximum speed. These methods generally become too unstable and oscillatory at lower speeds. Some methods remain stable at lower speeds but sacrifice the ability to respond to rapidly changing process conditions at low speeds.
The inability to control the material tension at low speeds results in a loss of tension control during the ramp up and ramp down phases of the process. Loss of control at these times results in undesirable material breaks, increased process waste, and lost productivity. The lack of adequate tension control at low speeds and also the absence of adequate control system response to changes in the modulus of elasticity of the material being processed also results in non-uniform finished products that must be disposed of as waste.